According to their nature shiftable transmissions are used particularly in mobile excavators, loaders and similar machinery. The individual gear steps of the mechanical transmission are alternatively engageable and disengageable via respective friction clutches. At least two of the friction clutches are designed as spring accumulator clutches and can be controlled in an opening direction by oil pressure produced by a pressure medium source. A friction clutch is preferably designed as a brake and has a static oil supply free of oil leakage.
Such a shiftable transmission has been disclosed in EP 0 408 592 B1 where a first actuation device can be actuated by pressurization in an opening direction and in a closing direction by unpressurized shifting, via a spring force, and a second actuation device can be actuated by pressurization in an opening direction and in a closing direction by unpressurized shifting, via a spring force, both actuation devices being disposed coaxially to each other. If both actuation devices are shifted without pressure and both clutches are closed, the output of the transmission is blocked, whereby a parking brake position is attained. The spring force of the first actuation device is laid out so that the clutch, belonging to the second actuation device, can transmit the needed torque. If the parking brake is engaged while the output of the transmission is rotating, that is, when the vehicle is moving, then the fist and second actuation devices are shifted without pressure whereby both clutches are actuated in closing direction by the spring force. This is also attained when, due to a defect, the pressure medium source cannot deliver the needed pressure level or the pressure medium supplies have been damaged. Since the spring forces of the actuation devices are laid out so that they can transmit the needed torques, there is obtained in such a deceleration too great a delay whereby the vehicle can encounter uncontrolled situations.
EP 0 466 742 B1 discloses a shiftable transmission in which two clutches can be actuated by pressurization of the actuation devices in an opening direction and by unpressurized shifting of the actuation devices in a closing direction in which case, when both actuation devices are shifted without pressure, the output of the transmission can be decelerated, a pressure medium source additionally supplying an accumulator with pressure medium which, in case of such deceleration, supplies a first actuation device with pressure medium until the transmission has decelerated the vehicle under control. Such a supply of the actuation devices with pressure medium is very wasteful and expensive. Since in said transmission only one actuation device has one static oil supply and the oil supply to the second actuation device is carried out dynamically, it is necessary to ensure that, in case of an emergency deceleration, the accumulator is always connected with the first actuation device. Due to the arrangement in a transmission of a static and a dynamic pressure medium supply, the transmission has considerably more parts and places to seal.
EP 0 659 250 B1 and EP 0 745 199 B1 disclose a shiftable transmission with spring accumulator deceleration in which the actuation devices can be pressurized via a throttle recoil valve whereby the actuation devices can be actuated quickly in opening direction, but the closing operation develops slowly. It is hereby ensured that one clutch always opens precisely before the second clutch is closed.
The problem on which this invention is based is to provide a shiftable transmission which stands out by a simple construction of the actuation devices and eliminates expensive hydraulic controls.